Laying cement board

I'll repeat: You should be sure to scrape any purging thinset from the edges of the boards. For sure don't leave it overnight then forget to do it the next day.

When thinset comes in contact with cement board the cement board rapidly wicks the moisture from the thinset and the thinset stiffens. If the purging thinset along the edges is allowed to remain, this is what causes lippage in adjacent cement boards. Keep up with it and it won't become an issue later. Even the short time it takes to go get the next board is sometimes long enough for the purge to stiffen and cause the edge of the next board to stand proud of the first.